Mossy fibers enter the cerebellum via all three cerebellar peduncles and provide collateral fibers to the cerebellar nuclei before heading up to the cortex. In sum, via the mossy fiber system, the cerebellum receives a wide variety of sensory information, as well as descending motor-related and cognitive-related activity. In contrast to the diverse origins of mossy fibers, olivocerebellar fibers all originate from a single nucleus: the inferior olivary nucleus, which is located in the rostral medulla, just dorsal and lateral to the pyramids. Almost all the olivary neurons are projection cells whose axons leave the nucleus without giving off collaterals and then cross the brainstem to enter the cerebellum primarily via the inferior cerebellar peduncle. Like mossy fibers, olivocerebellar axons are excitatory and send collaterals to the cerebellar nuclei as they ascend through the cerebellar white matter to the cortex. In the cerebellar cortex, olivocerebellar axons may synapse with basket, stellate, and Golgi cells, but they form a special synaptic arrangement with Purkinje cells. Each Purkinje cell receives input from only a single climbing fiber, which “climbs” up its proximal dendrites and makes hundreds of excitatory synapses. (The terminal portion of the olivocerebellar axon is referred to as a climbing fiber.) Conversely, each olivary axon branches to form about 10 to 15 climbing fibers.
CHAPTER 9 Organization of Motor Function •Fig. 9.20 Anatomic Divisions of the Cerebellum. A, Schematicmidsagittalviewofthefoldingofthecortexintolobe,lobules,andfolia.B, Schematicviewofanunfoldedferretcerebellarcortextoillustrateearliercompartmentationschemesforsubdividingthecerebellarcortexintothree(vermis,paravermis,andhemisphere)andthensevenlongitudinallyrunningzones(A,B,C1,C2,C3,D1,D2).Thelight yellow portionofeachhemisphereindicatesanareaforwhichnodatawereavailable.